This project investigates the actions of two homeodomain transcription factors that help control expression of nearly all of the more than 1,000 mouse odorant receptor genes, expression so finely regulated that each olfactory nerve cell expresses just one allele of one odorant receptor gene. The overarching hypothesis for the project proposes that homeodomain transcription factor stimulation of odorant receptor gene transcription triggers local positive feedback acting through active chromatin modifications while simultaneously enhancing global negative feedback acting through repressive chromatin modifications, competing events capable of explaining the singularity of odorant receptor expression. This project tests whether the transcription factors act directly at promoters and enhancers of odorant receptor genes, and whether these events are associated with selected active chromatin modifications at odorant receptor genes.